FIG. 3 illustrates the manner in which the state of rise of a stepped waveform is improved by a known edge stressing method, disclosed in an article entitled as "A Method of Computing Sharpness of Television Image" appearing in the Gazette of Society of Electro-Communication Engineering" (July 1983, Vol. J66-B No. 7).
It is well known that the sharpness of a television image can be improved by enhancing contour portions of the image, and this method is widely applied to luminance signals which determine the shape of an object.
A typical known correction means capable of enhancing contour portions of an image is arranged, as shown in FIG. 3, such that a signal obtained by quadratic differentiation of an original signal waveform is multiplied with a certain coefficient and the product of the multiplication is added to the original signal waveform.
Referring to this Figure, more specifically, a stepped waveform I.sub.2 (=Q(x)), which is an original signal obtained through a band restriction in a transmission path, is subjected to a quadratic differentiation so that a quadratic-differentiated waveform Q"(x) is obtained. Then, the quadratic-differentiated waveform Q"(x) is multiplied with a coefficient -a so that a waveform Id is obtained. The signal Id thus obtained is added to the original signal waveform I.sub.2, whereby a contour correction signal waveform I.sub.2 +Id is obtained.
The amount of improvement in the rise time achieved by this contour correction means is expressed in terms of time relative to the amount of change in the amplitude. In the case of the original waveform I.sub.2, the amount of improvement is expressed as V.sub.1 /X.sub.1, whereas, in the case of the contour correction signal waveform I.sub.2 +Id, the amount of improvement is V.sub.2 /X.sub.2. It will be seen that the rise time is improved appreciably.
The improvement in the rise of signals corresponding to the contour portions of a television image, i.e., enhancing of contour portions, provides a higher sharpness of the image.
In the known contour correction means, the correction signal is obtained by determining the quadratic-differentiated value of the original waveform, multiplying this value with a coefficient, and adding the product of the multiplication to the original waveform. Thus, as represented by hatched portions in FIG. 3, the level of the correction signal waveform exceeds the level of the original signal waveform so that saturation of the waveform may take place when the amplitude of the original waveform coincides with the maximum amplitude of the dynamic range of the transmission path.